The present invention relates to methods and apparatus for heat treating metallic material, and more particularly the invention relates to a method and apparatus wherein a metallic material to be heat treated, e.g., a long material such as steel pipe, steel bar, shape steel, rail or the like is conveyed in its lengthwise direction at a stable speed, thereby preventing the occurrence of non-uniform heat treatment and ensuring improved heat treating efficiency.
In the past, where a long metallic material such as represented by large diameter steel pipe or the like is subjected to heating treatment over the entire length thereof by continuously conveying the material through a relatively short heating zone, it has been the usual practice to ensure that the material is fed at a constant speed as far as possible so as to prevent non-uniform heating of the parts of the material, that where the heating is effected by induction heating thus causing the ends of the material to tend to be underheated, preliminarily a dummy is joined to each end or excess length for cutting allowance is provided at each end so as to cut off the same after heat treatment, that straightener rolls are provided to remove any distortions produced in the material during the heat treatment, and so on.
In particular, where large diameter steel pipe or the like is heat treated with an induction heating coil, due to the fact that the length of the induction heating coil is extremely small as compared with the length of the metallic material to be heat treated, it is an essential requirement that the metallic material is moved through the coil at a constant speed at all times. In other words, if the travel speed of a metallic material through the coil differs for different positions of the material, this causes the heating time to differ for the different positions of the material with the result that even if the amount of heat supplied per unit time is made constant by the electric heating method. i.e., induction heating (even if the uniform heating of the metallic material in a plane normal to the directon of travel of the material is made easier), the metallic material cannot be heated uniformly in time and consequently the different parts of the material subjected to heating process will be heated to different temperatures, that is, the temperatures of these parts will not be uniform. Of course, this tendency becomes more marked with increase in the preset heating temperature of the coil.
If a metallic material to be subjected to heating treatment is exposed to different heating conditions locally (in the lengthwise direction), the following problems will be caused. In other words, firstly the mechanical properties of the heat treated material will not be the same throughout the material. Secondly, change of shape will be caused by the non-uniform heating. These phenomena will become more marked with increase in heating temperature and increase in the non-uniformity of heating due to the irregularity in the travel speed of the material, and these phenomena also become more marked with increase in the cooling rate for the cooling process following the heating process. As a result, where the material is heated to a relatively high temperature and then cooled rapidly as during a quenching treatment, these phenomena will be still more marked. Up to date, the following conveying methods have been used in connection with the heat treatment by induction heating of such metallic material as large diameter steel pipe.